The invention relates to a device for measuring the flow or pressure of a fluid. The fluid passes through a primary element in which the pressure on respective sides of a restriction in the primary element acts on respective sides of a diaphragm mounted in a diaphragm housing. The diaphragm, in turn, acts on a measuring element provided with strain gauges and connected to a measuring bridge which indicates the flow through the primary element. In pressure measurements, the fluid pressure acts on one side of the diaphragm only.
In previously known flowmeters based on the strain gauge principle, such as in U.S. Pat. No. 2,707,393, which is designed to measure whether and to what degree a flow is pulsating, the movements of the diaphragm are transferred to the body housing and the strain gauges by means of a linkage, which gives rise to unwanted hysteresis effects as the flow increases and decreases. Since the strain gauges are in direct contact with the fluid, the results of measurement are also affected by changes in fluid temperature.
The above is true, for example, of U.S. Pat. No. 3,444,736. Here, a complicated arrangement is disclosed wherein the diaphragm, which actuates the strain gauges, is surrounded by silicone oil and is enclosed between two further diaphragms which, in turn, detect the pressure differential in a venturi tube. This device also causes unwanted hysteresis effects and, according to the disclosure of this patent, affords a flow ratio of only 3:1. Furthermore, in both of these devices, the electrical connections from the strain gauges, which are immersed in pressurized fluid, must be provided with fully sealed electrical penetrations.
It is an object of the invention to eliminate the disadvantages associated with previously known devices and to provide a flowmeter of simple construction which is entirely free of hysteresis effects as the flow increases and decreases.
The flowmeter of the invention is for measuring the flow of fluid flowing through a primary element. The flowmeter includes: a restriction mounted in the primary element through which the fluid flows thereby generating a differential pressure across the restriction; a housing; a diaphragm mounted in the housing; the housing being connected to respective sides of the restriction so as to permit the differential pressure to act on the diaphragm; a diaphragm spindle attached to the diaphragm so as to be displaced in response to changes in the differential pressure; a measuring element defining a hollow space; a finger arranged in the hollow space so as to extend transversely to the diaphragm spindle; the finger being fixedly connected to the measuring element at the lower end of the hollow space; connecting means for connecting the diaphragm spindle to the finger for transmitting the movements of the diaphragm to the finger which, in turn, imparts deflections to the measuring element; the measuring element including strain gauges mounted thereon for sensing the deflections transmitted to the measuring element via the finger; and, a measuring bridge incorporating the strain gauges to provide an electrical signal indicative of the flow through the restriction.
Measured values, which are completely independent of temperature changes in the fluid, are provided by the flowmeter of the invention because the strain gauges are located completely separate from the fluid and are mounted symmetrically on a measuring element. A further advantage is that the fluid cannot damage the strain gauges through its aggressivity, toxicity, temperature, pressure or abrasive properties. This is achieved by an arrangement in which a diaphragm mounted in a diaphragm housing is acted on by the higher and lower pressures present at respective sides of a primary element, and in which a diaphragm spindle extends at right angles to the diaphragm proper, one end of the spindle being attached to the center of the diaphragm and the other end being provided with a free-fit hole. One end of a finger is held, without play, by an adjusting screw located centrally in the hole in the free end of the diaphragm spindle. The finger extends downwardly through a hollow measuring element, to which it is fixed at the lower end. The measuring element, the hollow interior of which is open to the fluid on one side of the diaphragm, is bolted to one side of the diaphragm housing in a fluid-tight manner. Since the diaphragm tends to move towards the side on which the lower pressure acts, it exerts a pulling or pushing action on the diaphragm spindle depending on the side on which the high pressure is allowed to act. The diaphragm spindle, in turn, deflects the measuring element via the finger attached to it. Strain gauges bonded to the outside of the measuring element are connected, in a conventional manner, to a Wheatstone bridge, which converts the changes in resistance of the strain gauges into an electrical signal. This signal can be used to provide a flow-calibrated reading on an indicating instrument and/or otherwise to record changes in flow or to control the flow at a desired value with the aid of a known control system. The invention also affords a flow ratio of 10:1 or 100:1 when measuring pressure only, which is significantly higher than that of previously known devices.